In electrophotography, as set forth in various disclosures, for example, U.S. Pat. No. 2,297,691 and JP-B No. 42-23910 (hereinafter, the term, JP-B refers to Japanese Patent Publication), electrostatic latent images are generally formed on a photoreceptor containing a photoconductive material through various means, the formed latent image is developed with a toner to form a powdery image and the powdery image is optionally transferred onto paper or the like, then, the powdery image is fixed by applying heat, pressure or solvent vapor.
Recently, color copying was put to practical use, in which electrostatic latent images of an original are formed through spectral light exposure and are developed with an individual color toner to obtain a colored copy image. Alternatively, respective color copy images are superimposed to obtain a full color copy image. As color toners used therein, there are manufactured color toners of yellow, magenta, cyan and the like, formed of an individual color pigment and/or dye dispersed in a binder resin.
In the foregoing electrophotography, image formation is generally performed according to the following steps. First, light information corresponding to image information is exposed onto a photoreceptor comprised of a photoconductive material through various methods to form an electrostatic latent image on the photoreceptor. Then, the electrostatic latent image formed on the photoreceptor is developed with a charged toner to form a toner image. The toner image is transferred onto an image recording medium (which is usually paper or an intermediate transfer material). The transferred image is fixed on plain paper using a thermal fixing apparatus. In the image forming method using electrophotography as described above, an electrostatic latent image formed on the photoreceptor corresponds to image information separated to an individual colors of yellow, magenta, cyan and black and are developed with a toner having the same color as the respective image data. The development is repeated for each color to form a full color image, namely up to four repetitions.
Commonly known organic pigments and dyes have been used as coloring material used for electrophotographic toners but they exhibit various defects. For instance, organic pigments, compared to dyes, are generally superior in heat resistance and light resistance, and exist in a toner in the form of a particle dispersion, resulting in enhanced covering power, leading to lowered transparency. Dispersibility of a pigment is generally poor so that transparency is vitiated and hue is lowered, resulting in deteriorated color reproduction of images.
To make it possible to visually recognize color of the lowest layer of overlaid toner layers without being concealed by upper toner layers, transparency of a fixed toner is required, and dispersibility or coloring power of a coloring agent is needed to maintain color reproducibility of the original.
To overcome defects of pigments as described above, there were proposed, for example, a means for enhancing transparency whereby primary particles exhibited a pigment dispersion diameter of the sub-micrometer order without forming aggregated secondary particles, by using a flashing method as a means for dispersing a pigment, as disclosed in JP-A No. 5-11504 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication) and also a means for improving the electrostatic-charging property, fixability and image uniformity by covering pigment particles with a binding resin and a shelling resin, as disclosed in JP-A No. 11-160910. However, even when outputted by using the proposed toners, it is difficult to obtain sufficient transparency in the case of a toner using pigments.
In a color image forming apparatus, all of color reproduction is theoretically feasible through subtractive mixture of the three primary colors of yellow, magenta and cyan. However, the color-reproducible region or hue is lowered by spectral characteristics of a pigment dispersed in a thermoplastic resin or by color mixing property when overlaying different color toners, so that there still remain many problems to achieve faithful color reproduction of an original.
There were also disclosed toners using dyes, as described in JP-A No. 5-11504 and toners using a mixture of dyes and pigments. In a toner using a dye, the dye exists in a state of being dissolved in a binding resin for the toner, resulting in superior transparency and hue but exhibiting defects such as inferior light resistance and heat resistance of the pigments. With respect to heat resistance, in addition to lowering in density due to degradation of a dye, problems were produced such that the dye sublimed and easily stained the machine and additionally the dye was dissolved in silicone oil used for fixing and finally melted onto the heated roller, causing the off-setting phenomenon while fixing toner images by a heated roller. To overcome such defects of dyes, there were proposed, for example, a technique of using specific anthraquinone type dyes or chelate dyes for a magenta toner to allow light resistance or sublimation property to be compatible with color reproduction, as described in JP-B No. 3567403 and an encapsulated toner in which a core including a polymer resin and a color dye was covered with a polymer, as described in JP-A No. 5-72792.
However, even when outputted using these proposed toners, it was difficult for such dye-using toners to achieve sufficient heat resistance (sublimation property) and light resistance. Further, some of the metals, for example, nickel or cobalt, used in chelate dyes included problems with respect to safety. Accordingly, there was desired development of a toner meeting these conditions.